Disks are generally flat inside radius R25. Out to radius
R26.5 the line of nodes of a warp is straight
Implication: Self-gravity of the disk is important
(it keeps the different parts of the disk precessing
synchronously and hence the line of nodes straight - cf. the
winding problem of spiral waves)

The outer line of nodes advances in the direction of galactic rotation
Implication: Warps are not quite in equilibrium at large
radii. This points to a link to the environment, or to very
long timescales

Warps are primarily observed in HI. Significant warps (misalignments
between inner and outer parts of the disk of more than a degree or
two) in stellar disks are rarely observed
(Reshetnikov &
Combes 1999),
though recently the warp of the Milky Way has also been seen in the stellar
distribution
(Alard 2000).
This may indicate that warps are a
phenomenon which affects only the cold ISM, or that only the very
outskirts of galaxy disks, which are only observable in HI, are
involved.

The Galaxy warp illustrates the general point that, analogously to
spiral structure, warps come in several varieties: `grand-design',
nice integral-sign shaped bi-symmetric warps; `irregular' warps which
are only visible on one side, or in which one side of the galaxy is
more warped than the other; and `feeble' (weak or absent) warps. To
determine the relative frequency of these classes is important, as it
points the way for attempts at an explanation: should we be looking
for mechanisms which naturally produce beautiful integral-sign warps,
or for - possibily more chaotic - ways to make irregular ones?

In order to address this question, we have performed a blind survey of
edge-on (as judged from optical images) galaxies. These galaxies form
part of the WHISP sample, which is basically an HI flux-density
limited sample (> 200mJy) selected from the UGC catalogue. We chose
galaxies with blue major diameters larger than 2', and inclination
class 6 or 7. DSS images of these galaxies were inspected to filter
out obviously less-inclined galaxies, to leave us with a sample of
galaxies which, as judged from optical images, have inclinations at
least
~ 80°. We chose edge-on galaxies for this survey so that
the warps can be studied purely morphologically, without the need for
interpretation or modelling of the velocity field (e.g., by means of
tilted ring models).

Our sample clearly shows that warps occur in all types. About 1/3 of
our galaxies show a nice integral-sign warp, about 1/3
are flat, and the final 3rd of the sample are of the irregular
(one-sided or asymmetric) types (Table 1).

Table 1.
The classification of warps in a sample of 28 edge-on
galaxies, observed in HI with Westerbork (Garcia-Ruiz et al., in
preparation). Note the large fraction of galaxies which do not fall in
the classical, integral-sign warp category.